Phenomenological description of semi-soft nematic elastomers

Nematic elastomers are new materials that have many remarkable properties. In this article, we study how nonlinear elasticity of semi-soft nematic elastomers can be described phenomenologically. We start with a theory based on strain tensor only, and then continue to develop a phenomenological description with the liquid crystal order tensor included explicitly. Such a description has the virtue of being able to treat the strain tensor and the liquid crystal order tensor equally and thus making the complicated symmetries of nematic elastomers easier to understand.